Engine idles too high?

The car starts fine and runs fine as well, what is happening is the car wants to push itself, it will go along at 30 kms/hr without touching the gas, if you slip it into neutral it revs @ 1800 rpm untill you come to complete stop, then the idle will go back down to @ 850 where it suppost to be, it shows no codes, the trottle body has been cleaned, we also replaced the air idle control as well, seems a little better but still wants to push itself along. My machanic is now sratching his head as well, again no codes come up, help please.